Medical ultrasound systems form images by sequentially acquiring the echo signals of adjacent ultrasound beams. An individual beam is formed by transmitting a focused pulse and receiving the echoes over a range of depths. During receive, the transducer delays are continuously updated such that the aperture is always focused to the depth from which the echo originated. This approach is commonly called dynamic receive focusing.
Transmit, being a single event, cannot be dynamically focused. The pulse, once launched, will only be focused at a single depth. This causes depths other than the focal point location to be out of focus to various degrees. Earlier imaging systems controlled this transmit focusing error by reducing the transmit aperture (i.e., increasing its F-number). The reduction in transmit aperture increased the depth of field to an acceptable level. This approach required a trade-off between the beamwidth at the focus and the depth of field. This single transmit focus approach is still the only successful imaging method when frame rate (or image update rate) is of highest importance. Cardiology and pediatric cardiology are the most obvious applications for this technique.
The trade-off between beamwidth and depth of field is undesirable because it limits spatial resolution. The approaches to overcome this deficiency can be grouped into multi-focus and synthetic aperture imaging. Most conventional medical ultrasound systems employ a form of multi-focus imaging. Here, the ultrasound beam is composed of several range segments. For every segment, the transmit and receive sequence is repeated. The transmit pulse is focused approximately in the middle of the segment's active range while reception is still dynamically focused. The echo signals originating from the active zone are retained for further processing while the other echoes are discarded. After all segments have been acquired and processed, the video detected echo signals are adjoined at so-called stitch lines.
The depth of field of a given transmit focal point needs to extend over only the length of the range segment. Therefore, the beamwidth to depth of field ratio trade-off can be shifted towards a narrower beam. Improved spatial resolution can be achieved by dividing the image into a larger number of segments.
Synthetic aperture imaging is a different approach, with two major benefits: a lower number of system processing channels and increased depth of field. U.S. Pat. No. 4,604,697 discloses an approach where the aperture is divided into a number of subapertures. Each subaperture independently acquires an image and those images, actually subimages, are added together to form the final image. The summation of subimages is done before any nonlinear processing, such as detection or compression. Since the summation is phase sensitive, an appropriate phase correction is applied to account for pixel location with respect to the various subaperture locations. Depth of field of the final image is determined by the depth of field of the subimage. Since each subaperture is only a fraction of the overall aperture, its depth of field is comparably large. With this method, a trade-off is made between the depth of field and the number of subapertures. Resolution improvement occurs in the phase-sensitive summation of subimages. Because the phase correction can be applied correctly for every pixel, a continuous transmit focus can be achieved.
An advantage of multi-focus imaging over synthetic aperture imaging is the independence of the various segment acquisitions. In synthetic imaging several phase sensitive subimages are added. Any target motion between the subimage acquisitions introduces phase errors, and thus, imaging artifacts. The phase errors are proportional to target motion and the acquisition time of all signals that contribute to a given pixel. Due to the inherent target motion in medical ultrasound, synthetic aperture imaging (in general) has not been successful. There is, however, an existing proof of synthetic imaging. A conventional ultrasound imaging system employs Golay-coded excitation wherein each scan line is formed by adding the echoes from two successive encoded transmissions. The signal summation is phase sensitive to cancel the range sidelobes of the individual transmissions. Phase errors due to target motion are acceptable because only two signals are added. Therefore, the acquisition time over which phase coherence is required remains comparably short. If synthetic aperture imaging were used to significantly increase the depth of field, a much larger number of subimage signals would be required.
A serious disadvantage of both multi-focus and synthetic aperture imaging is the increased time required to form an image frame. To a first approximation, this time is proportional to the number of range segments or subapertures. The frame rate is the inverse of the acquisition time. Due to the sequence of dependencies: acquisition time/number of segments/depth of field/beamwidth/spatial resolution, a trade-off must be made between frame rate and spatial resolution. For a system which favors spatial resolution, there is need to improve the frame rate.
Other approaches to improving frame rate are multi-line beamforming and RF (radio frequency) interpolation. The purpose of multi-line beamforming and RF interpolation is the recovery of the frame rate which is lost due to multiple transmit focal points. In multi-line beamforming, multiple (e.g., two) receive lines are acquired for one transmit pulse. To a first approximation, the frame rate increases by the factor of parallel receive lines. In practice, however, such improvement cannot be achieved because of acoustic artifacts associated with the multi-line acquisition. Due to those artifacts the line spacing must be decreased, which by itself reduces the frame rate. The most apparent artifact from multi-line acquisition is a lower spatial resolution due to the wider transmit pulse. A less focused transmit pulse is required to send acoustic energy into the multiple receive directions. Yet another artifact appears as an ultrasound line warping in the area of the transmit focal location. This is caused by narrowing of the transmit beam around its focus.
Radio frequency interpolation also increases the number of echo lines per transmit pulse. The ultrasound beams are acquired in the same way as in a conventional system (possibly including multi-focus). Then, however, additional RF lines are interpolated between the acquired lines from the beamformer. The interpolation is performed on the RF (or baseband I/Q) signals while the conventional scan interpolation operates on the detected signal. The RF interpolation includes phase information and therefore eliminates artifacts associated with video interpolation. Due to the artifact elimination, the physically acquired ultrasound lines (from the beamformer) can be spaced farther apart than in a conventional system and thus the frame rate can be increased.
The system implementation of multi-line acquisition is expensive since it requires parallel signal paths in the beamformer. Although the complexity increases almost linearly with the number of parallel receive lines, the amount of hardware increases less than that rate. Some of the parallel hardware can be efficiently implemented using custom designed chips, and thus some hardware savings can be attained. The implementation of RF interpolation is much less demanding since the beamformer is not affected. The RF interpolation uses beamsummed data of consecutive firings and interpolates one or more values for every sample.
One conventional ultrasonic imaging system achieves high spatial resolution by applying a low F-number in both transmit and receive. A drawback of the low transmit F-number is a short depth of field for a given transmit focal point, i.e., the high spatial resolution is achievable only over a small range in depth. To overcome this deficiency, the image is acquired in range segments where each segment has its dedicated focal point. By applying a large number of focal points, spatial resolution is improved. While this provides good image quality, it compromises on the imaging frame rate, i.e., the image acquisition time is drastically increased by the sequential processing of a large number of focal points. In a conventional ultrasound system, each range segment is acquired with its associated transmit focal point. The segments are processed individually and joined to form the image. An image point is formed from data from a single segment. (There might be a short transition to blend from one segment to the other; however, no attempt is made to combine data from more than one focal point with the goal of correcting for focusing errors.) For every additional image segment, a separate transmit event is required, thereby increasing the acquisition time for the entire image. As a first approximation, the acquisition time is proportional to the number of segments, i.e., the number of transmit focal points. There is need for a method which will allow the number of transmit focal points to be reduced without loss of spatial resolution.